So...here's a mental exercise -- what if we didn't? What if some biological weapon capped us all in the reproductive organs before the industrial age (like steampunk gone awry, where steam and mold give everyone killer allergies)? Then all the fossil fuels could have sit unburned, and then explosively detonated like the New York City sewer system, except on a planet scale.
Then where would we be?
We'd be extinct, that's where.